A laser beam defines a substantially perfectly straight line. It has been found extremely practical when building tall objects, making deep excavations, digging tunnels, and the like, to use a fixed laser beam as a reference to allow the work to be conducted along a given straight path. This system is particularly useful in the erection of tall buildings where a vertical laser beam is projected up at a slight distance outwardly from a side of the building. It is possible by means of a very simple measurement at any level to determine the deviation of the building from this vertical reference line.
This is typically achieved by positioning a projector on the ground at the bottom of the building being erected. During the original setup the device is meticulously adjusted so that the beam is projected or reflected perfectly vertically. Thereafter a laser receiver, or indeed even a simple yardstick, can be used to measure the horizontal distance between this perfectly vertical laser beam and the side of the building. Such a system is disclosed in German Patent publication 2,403,239, whose entire disclosure is herewith incorporated.
The difficulty with such a system is that the beam can become misaligned. This happens rather frequently in a building site due to the frequently unstable ground conditions and the considerable amount of work going on around the projector. In case of such misalignment it is necessary totally to realign the device and usually establish a new set of measurements for use after the realignment.